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Date:   Thu,  9 Nov 2017 17:18:53 +0800
From:   Peter Xu <peterx@...hat.com>
To:     kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>, peterx@...hat.com,
        Radim Krčmář <rkrcmar@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] sched/swait: allow swake_up() to return

Let swake_up() to return whether any of the waiters is waked up. One use
case of it would be:

  if (swait_active(wq)) {
    swake_up(wq);
    // do something when waiter is waked up
    waked_up++;
  }

Logically it's possible that when reaching swake_up() the wait queue is
not active any more, and here doing something like waked_up++ would be
inaccurate.  To correct it, we need an atomic version of it.

With this patch, we can simply re-write it into:

  if (swake_up(wq)) {
    // do something when waiter is waked up
    waked_up++;
  }

After all we are checking swait_active() inside swake_up() too.

CC: Ingo Molnar <mingo@...hat.com>
CC: Peter Zijlstra <peterz@...radead.org>
CC: Peter Xu <peterx@...hat.com>
CC: linux-kernel@...r.kernel.org
Signed-off-by: Peter Xu <peterx@...hat.com>
---
 include/linux/swait.h |  4 ++--
 kernel/sched/swait.c  | 21 ++++++++++++++++-----
 2 files changed, 18 insertions(+), 7 deletions(-)

diff --git a/include/linux/swait.h b/include/linux/swait.h
index c1f9c62a8a50..c55171280ac8 100644
--- a/include/linux/swait.h
+++ b/include/linux/swait.h
@@ -84,9 +84,9 @@ static inline int swait_active(struct swait_queue_head *q)
 	return !list_empty(&q->task_list);
 }
 
-extern void swake_up(struct swait_queue_head *q);
+extern bool swake_up(struct swait_queue_head *q);
 extern void swake_up_all(struct swait_queue_head *q);
-extern void swake_up_locked(struct swait_queue_head *q);
+extern bool swake_up_locked(struct swait_queue_head *q);
 
 extern void __prepare_to_swait(struct swait_queue_head *q, struct swait_queue *wait);
 extern void prepare_to_swait(struct swait_queue_head *q, struct swait_queue *wait, int state);
diff --git a/kernel/sched/swait.c b/kernel/sched/swait.c
index 3d5610dcce11..31f8d677c690 100644
--- a/kernel/sched/swait.c
+++ b/kernel/sched/swait.c
@@ -16,29 +16,40 @@ EXPORT_SYMBOL(__init_swait_queue_head);
  * If for some reason it would return 0, that means the previously waiting
  * task is already running, so it will observe condition true (or has already).
  */
-void swake_up_locked(struct swait_queue_head *q)
+bool swake_up_locked(struct swait_queue_head *q)
 {
 	struct swait_queue *curr;
 
 	if (list_empty(&q->task_list))
-		return;
+		return false;
 
 	curr = list_first_entry(&q->task_list, typeof(*curr), task_list);
 	wake_up_process(curr->task);
 	list_del_init(&curr->task_list);
+	return true;
 }
 EXPORT_SYMBOL(swake_up_locked);
 
-void swake_up(struct swait_queue_head *q)
+/**
+ * swake_up - Wake up one process on the waiting list
+ * @q:        the waitqueue to wake up
+ *
+ * Returns true if some process is waked up, otherwise false if there
+ * is no waiter to wake up.
+ */
+bool swake_up(struct swait_queue_head *q)
 {
+	bool ret;
 	unsigned long flags;
 
 	if (!swait_active(q))
-		return;
+		return false;
 
 	raw_spin_lock_irqsave(&q->lock, flags);
-	swake_up_locked(q);
+	ret = swake_up_locked(q);
 	raw_spin_unlock_irqrestore(&q->lock, flags);
+
+	return ret;
 }
 EXPORT_SYMBOL(swake_up);
 
-- 
2.13.6

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